The present invention relates to improvements in interferometer stabilizing circuits and more particularly to a new system and technique for maintaining the parallelism of optical surfaces in a swept interferometer.
Generally, swept interferometers include two optical surfaces spaced from one another along a collinear axis and positioned such that the surfaces are substantially parallel. Light from the interferometer varies in intensity as the distance between the optical surfaces is changed by sweeping one of the surfaces relative to the other along the collinear axis. The surfaces are generally controlled by the application of control voltages to piezoelectric mounting structures supporting the optical surfaces with optimal performance being obtained when the surfaces are substantially parallel. Over a period of time, from a few minutes to a few hours, the surfaces will become non-parallel due to temperature drifts, mechanical features of the piezoelectric mountings, fluctuations in positioning control voltages, and non-linear properties, such as hysteresis in the piezoelectric elements. Even if temperature can be carefully controlled, which is not an inexpensive or simple matter, the other instabilities still degrade the performance of the interferometer.
Various techniques have been utilized to prevent the surfaces from becoming non-parallel but each has met with problems limiting extensive application of such devices to interferometer systems. In one such system a photomultiplier tube detects light from an interferometer and delivers a voltage to storage capacitors. Gating signals, synchronized with the sweeping voltage of the interferometer, then control application of the voltage to piezoelectric transducers to correct for non-parallel surfaces. While this technique has been successful in compensating for temperature variations, electrical adjustments during operation are required in order to insure accurate operation over long periods of time.
In still another technique, plural light beams are used to develop control signals which indicate parallelism of the surfaces by modulation of an electrical signal from the changes in intensity of the light beams. This particular technique requires a complex arrangement of prisms and phase detectors in order to control the surface parallelism and therefore introduces still other parameters that may require substantial adjustments during operation.
While the above techniques have suggested methods for improving performance of interferometers in limited applications, there is still a continuing need for control devices that will provide accurate compensation for mechanical and temperature changes in interferometer systems. Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a more reliable and versatile system for maintaining the parallelism of optical surfaces.